Re‐Investigation of Hydration Potential of Rhodococcus Whole‐Cell Biocatalysts towards Michael Acceptors

The implementation of a stereoselective Michael addition with water as substrate is still a major challenge by classical, chemical means. Inspired by nature's ability to carry out this attractive reaction with both high selectivity and efficiency, the interest in hydratases (EC 4.2.1.x) to accomplish a selective water addition is steadily rising. The gram‐positive bacterial genus Rhodococcus is known as biocatalytic powerhouse and has been reported to hydrate various Michael acceptors leading to chiral alcohols. This study aimed at the in‐depth re‐investigation of the hydration potential of Rhodococcus whole‐cells towards Michael acceptors. Here, two concurrent effects responsible for the hydration reaction were found: while the majority of substrates was hydrated in an oxygen‐independent manner by amino‐acid catalysis, an enzyme‐catalysed water addition to (E)‐4‐hydroxy‐3‐methylbut‐2‐enoic acid was proven to be oxygen‐dependent. 18O2‐labelling studies showed that no 18O2 was incorporated in the product. Therefore, a novel O2‐dependent hydratase distinct from all characterised hydratases so far was found. Whole‐Cell Biocatalysis: The net microbial water addition to Michael acceptors catalysed by Rhodococcus whole‐cells was re‐evaluated. 18O2‐ and D2O labelling studies excluded an oxidative process while confirming a true water addition. Depending on the substrate, the water addition is amino acid‐catalysed or performed by an oxygen‐dependent novel hydratase.